Make zsockopt_set_... take const char* args

Currently the functions
```
zsockopt_set_subscribe()
zsockopt_set_unsubscribe()
zsockopt_set_identity()
```
take (non-const) char* arguments. zmq_sockopt however takes the option value as const void*.

This pull request changes the CZMQ functions to accept const char*.
This allows constructs like zsockopt_set_subscribe(socket, "myfilter").

AFAIK this change doesn't introduce incompatibilities because the files are pure C (not C++), and therefore the symbol names are not mangled (objdump on zsockopt.o confirms this).

Please feel free to comment/discuss!